NARA
General John B. Magruder's Papers, 1874 - 1899
National Archives Identifier: 599429
ARC Identifier: 599429
Inventory Entry Number: 128
Entry 128 is located in PI101, "Preliminary Inventory of the War Department Collection of Confederate Records" (RG 109).
HMS/MLR Entry Number: PI101 128
Also, Magruder's military correspondence can be traced through NARA's War Department Collection of Confederate Records" (RG 109). Many of these documents are now digitized.